Thin sheets made of an aluminum-magnesium-scandium alloy for aerospace applications

ABSTRACT

The invention relates to a method for producing a wrought product made of an aluminum alloy composed, in wt %, of Mg: 3.8-4.2; Mn: 0.3-0.8 and preferably 0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.01-0.05; Zr: 0.07-0.15; Cr: &lt;0.01; Fe: &lt;0.15; Si&lt;0.1; wherein the homogenization is carried out at a temperature of between 370° C. and 450° C., for between 2 and 50 hours, such that the equivalent time at 400° C. is between 5 and 100 hours, and the hot deformation is carried out at an initial temperature of between 350° C. and 450° C. The invention also relates to hot-worked products obtained by the method according to the invention, in particular sheets with a thickness of less than 12 mm. The products according to the invention are advantageous as they offer a better compromise in terms of mechanical strength, toughness and hot-formability.

FIELD OF THE INVENTION

The invention relates to a method for producing wrought products made of an aluminum-magnesium alloy, also known as a SXXX series aluminum alloy according to the Aluminium Association, more particularly Al—Mg alloy products containing Sc having a high mechanical strength, high toughness and good formability. The invention further relates to products obtainable by said method, as well as to the use of these products intended for transportation and in particular for aircraft and spacecraft construction.

PRIOR ART

Wrought products made of an aluminum alloy are developed in particular to produce structural elements intended for the transportation industry and in particular for the aeronautics industry and the aerospace industry. In these industries, product performance must be constantly improved and new alloys are developed in particular in order to provide a high mechanical strength, low density, high toughness, excellent corrosion resistance and very good formability. In particular, forming can take place under heat, for example by creep forming, and the mechanical properties must not deteriorate after this forming process.

Al—Mg alloys have been extensively studied in the transportation industry, in particular that of road and sea transportation, due to the excellent properties thereof for use in such industries, such as the weldability, corrosion resistance and formability thereof, in particular in low-worked tempers such as the 0 temper and Hill temper.

However, these alloys have a relatively low mechanical strength for the aeronautics industry and aerospace industry.

U.S. Pat. No. 5,624,632 discloses an alloy composed of 3-7 wt % magnesium, 0.03-0.2 wt % zirconium, 0.2-1.2 wt % manganese, up to 0.15 wt % silicon and 0.05-0.5 wt % of an element forming dispersoids in the group consisting of scandium, erbium, yttrium, gadolinium, holmium and hafnium.

U.S. Pat. No. 6,695,935 discloses an alloy composed, in wt %, of Mg 3.5-6.0, Mn 0.4-1.2, Zn 0.4-1.5, Zr max. 0.25, Cr max. 0.3, Ti max. 0.2, Fe max. 0.5, Si max. 0.5, Cu max. 0.4, and one or more elements in the group: Bi 0.005-0.1, Pb 0.005-0.1, Sn 0.01-0.1, Ag 0.01-0.5, Sc 0.01-0.5, Li 0.01-0.5, V 0.01-0.3, Ce 0.01-0.3, Y 0.01-0.3, and Ni 0.01-0.3.

Patent application WO 01/12869 discloses an alloy composed, in wt %, of 1.0-8.0 wt % Mg, 0.05-0.6 wt % Sc, 0.05-0.20 wt % Hf and/or 0.05-0.20 wt % Zr, 0.5-2.0 wt % Cu and/or 0.5-2.0 wt % Zn and additionally 0.1-0.8 wt % Mn.

Patent application WO2007/020041 discloses an alloy composed, in wt %, of Mg 3.5 to 6.0, Mn 0.4 to 1.2, Fe<0.5, Si<0.5, Cu<0.15, Zr<0.5, Cr<0.3, Ti 0.03 to 0.2, Sc<0.5, Zn<1.7, Li<0.5, Ag<0.4, optionally one or more elements forming dispersoids in the group consisting of erbium, yttrium, hafnium, and vanadium, each <0.5 wt %. The products described in these patents are not sufficient in terms of offering a compromise between mechanical strength, toughness and hot-formability. In particular, it is important that the mechanical properties do not deteriorate after heat treatment at 300-350° C., which is a typical temperature for forming.

There is thus a need for wrought products made of an Al—Mg alloy with a low density and improved properties compared to those of known products, in particular in terms of mechanical strength, toughness and hot-formability. Moreover, such product must be obtainable according to a reliable and cost-effective production process that can be easily adapted to a conventional production line.

PURPOSE OF THE INVENTION

The invention firstly relates to a method for producing a wrought product made of an aluminum alloy wherein:

-   -   a) a molten metal bath having an aluminum base is produced,         composed, in wt %, of         -   Mg: 3.8-4.2;         -   Mn: 0.3-0.8; preferably 0.5-0.7;         -   Sc: 0.1-0.3;         -   Zn: 0.1-0.4;         -   Ti: 0.01-0.05, preferably 0.015-0.030;         -   Zr: 0.07-0.15, preferably 0.08-0.12;         -   Cr: <0.01;         -   Fe: <0.15;         -   Si<0.1;         -   other elements <0.05 each and <0.15 combined, the remainder             being aluminum;     -   b) an unwrought product is cast from said metal bath;     -   c) said unwrought product is homogenized at a temperature that         lies in the range 370° C. to 450° C., for a duration that lies         in the range 2 to 50 hours such that the equivalent time at         400° C. lies in the range 5 to 100 hours, the equivalent time         t(eq) at 400° C. being defined by the formula:

${t({eq})} = \frac{\int{{\exp \left( {{- 29122}/T} \right)}{dt}}}{\exp \left( {{- 29122}/T_{ref}} \right)}$

-   -    where T is the current temperature expressed in Kelvin, which         changes over time t (in hours) and Tref is a reference         temperature of 400° C. (673 K), t(eq) being expressed in hours,         the constant Q/R=29122 K being derived from the activation         energy for the diffusion of Zr, Q=242000 J/mol,     -   d) the unwrought product thus homogenized is hot-worked with an         initial temperature in the range 350° C. to 450° C. and is         optionally cold-worked;     -   e) a flattening and/or straightening process is optionally         carried out;     -   f) an annealing process is optionally carried out at a         temperature that lies in the range 300° C. to 350° C.

The invention secondly relates to a wrought product made of an aluminum alloy having the composition, in wt %,

Mg: 3.8-4.2;

Mn: 0.3-0.8, preferably 0.5-0.7;

Sc: 0.1-0.3;

Zn: 0.1-0.4;

Ti: 0.01-0.05, preferably 0.015-0.030;

Zr: 0.07-0.15, preferably 0.08-0.12;

Cr: <0.01;

Fe: <0.15;

Si<0.1;

other elements <0.05 each and <0.15 combined, the remainder being aluminum;

obtainable by the method according to the invention.

DESCRIPTION OF THE INVENTION

Unless specified otherwise, all of the indications concerning the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. By way of example, the expression 1.4 Cu means that the copper content expressed in wt % is multiplied by 1.4. The designation of the alloys is provided in accordance with the regulations of The Aluminium Association, known to those skilled in the art.

The definitions of the tempers are indicated in European standard EN 515 (1993). The tensile static mechanical properties, in other words the ultimate tensile strength R_(m), the tensile yield stress at 0.2% elongation R_(p0.2), and the elongation at rupture A %, are determined by a tensile test according to standard NF EN ISO 6892-1 (2009), whereby the sampling and the direction of the test are defined by standard EN 485-1 (2016).

The plane strain toughness is determined by a curve of the stress intensity factor KR as a function of the effective crack growth Δa_(eff) known as the R-curve, according to standard ASTM E 561 (2010). The critical stress intensity factor K_(C), in other words the intensity factor that makes the crack unstable, is calculated from the R-curve. The stress intensity factor K_(CO) is also calculated by assigning the initial crack length to the critical load, at the start of monotonic loading. These two values are calculated for a specimen of the required form. K_(app) represents the factor K_(CO) corresponding to the specimen that was used to carry out the R-curve test. K_(eff) represents the factor K_(C) corresponding to the specimen that was used to carry out the R-curve test. K_(R)60 corresponds to the value of KR for an effective crack growth Δa_(eff)=60 mm.

Within the scope of the invention, the grain structure of the samples is characterized in the plane LxTC at mid-thickness, t/2, and is quantitatively assessed after metallographic etching of the anodic oxidation type under polarized light:

-   -   the term “essentially non-recrystallized” is used when the grain         structure has no or few recrystallized grains, generally less         than 20%, preferably less than 15% and more preferably less than         10% of the grains are recrystallized;     -   the term “recrystallized” is used when the grain structure has a         significant proportion of recrystallized grains, generally more         than 50%, preferably more than 60% and more preferably more than         80% of the grains are recrystallized.

Unless specified otherwise, the definitions of standard EN 12258-1 (1998) apply.

Within the scope of the present invention, a “structural element” of a mechanical construction means a mechanical part for which the static and/or dynamic mechanical properties are particularly important to the performance of the structure and for which a structural calculation is usually prescribed or carried out. These are generally elements whose malfunction is likely to jeopardize the safety of said construction, of its users or of other persons. For an aircraft, these structural elements in particular include the elements that comprise the fuselage (such as the fuselage skin, fuselage stiffeners or stringers, bulkheads, circumferential frames, wings (such as the upper or lower wing skin), stringers or stiffeners, ribs, spars, floor beams and seat tracks) and the tail unit in particular comprised of horizontal or vertical stabilizers, as well as the doors.

The inventors hereof have observed that, for a composition according to the invention, an advantageous wrought product can be obtained by controlling the homogenization conditions, the mechanical properties of which advantageous wrought product offer a compromise between mechanical strength and useful toughness for the aircraft construction industry, and the properties whereof are stable after heat treatment corresponding to hot-forming conditions.

According to the invention, a molten metal bath having an aluminum base is produced composed, in wt %, of Mg: 3.8-4.2; Mn: 0.3-0.8, preferably 0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.01-0.05, preferably 0.015-0.030; Zr: 0.07-0.15, preferably 0.08-0.12; Cr: <0.01; Fe: <0.15; Si<0.1; other elements <0.05 each and <0.15 combined, the remainder being aluminum.

The composition according to the invention is noteworthy as a result of the low quantity of added titanium from 0.01-0.05 and preferentially from 0.015 to 0.030 wt % and preferably from 0.018 to 0.024 wt % and as a result of the absence of added chromium, the content whereof is less than 0.01 wt %. The high static mechanical properties (Rp0.2, Rm) are obtained despite these small additions, as a result of the carefully-controlled homogenization conditions. Thus, surprisingly, recrystallisation can be prevented during the hot-forming process with low quantities of added titanium and without added chromium, while simultaneously procuring high static mechanical properties, which were in particular possible to obtain by adding high quantities of Cr and Ti, and a high toughness.

Mn, Sc, Zn and Zr must be added in order to obtain the desired compromise between the mechanical strength, toughness and hot-formability. The iron content is kept below 0.15 wt %, and preferably below 0.1 wt %. The silicon content is kept below 0.1 wt %, and preferably below 0.05 wt %. The presence of iron and silicon in excess of the aforementioned maximum values has a negative impact, in particular on toughness. The remaining elements are impurities, i.e. elements whose presence is unintentional, the presence whereof must be limited to 0.05% each and to 0.15% combined and preferably to 0.03% each and to 0.10% combined.

According to the invention, said unwrought product is homogenized at a temperature that lies in the range 370° C. to 450° C., for a duration that lies in the range 2 to 50 hours such that the equivalent time at 400° C. lies in the range 5 to 100 hours, the equivalent time t(eq) at 400° C. being defined by the formula:

${t({eq})} = \frac{\int{{\exp \left( {{- 29122}/T} \right)}{dt}}}{\exp \left( {{- 29122}/T_{ref}} \right)}$

wherein T is the current temperature expressed in Kelvin, which changes over time t (in hours) and Tref is a reference temperature of 400° C. (673 K), t(eq) being expressed in hours, the constant Q/R=29122 K being derived from the activation energy for the diffusion of the Zr, Q=242000 J/mol.

Preferably, the homogenization duration lies in the range 5 to 30 hours. Advantageously, the equivalent time at 400° C. lies in the range 6 to 30 hours.

A too low homogenization temperature and/or a too short homogenization duration does not allow for the formation of dispersoids to control recrystallisation. Surprisingly, when the homogenization temperature is too high and/or when the homogenization duration is too long, the properties obtained are unstable at the conventional hot-forming temperature of 300-350° C., in particular since the products recrystallize.

Hot working can be carried out immediately after homogenization without cooling to ambient temperature, whereby the initial hot working temperature must lie in the range 350 to 450° C. Alternatively, the unwrought product can be cooled to ambient temperature after homogenization and then reheated to an initial hot working temperature that lies in the range 350 to 450° C. In the case of reheating, the equivalent time at 400° C. during reheating must be kept low, generally less than 10%, compared to the equivalent time at 400° C. during homogenization.

During hot working, the temperature of the metal can, in some cases, rise, however the equivalent time at 400° C. during hot working must be kept low, generally less than 10%, compared to the equivalent time at 400° C. during homogenization. In any case, the temperature during hot working must preferably not exceed 460° C. and preferentially not exceed 440° C. After hot working, cold working can be carried out.

In a first embodiment, working is carried out by rolling in order to obtain a sheet metal. In this first embodiment, the final thickness of the sheet metal obtained is less than 12 mm.

In a second embodiment, working is carried out by extrusion in order to obtain a profile.

In a first embodiment, hot working is generally carried out until a thickness of about 4 mm is reached, then cold working is carried out for a thickness that lies in the range 0.5 to 4 mm.

After the hot- and optionally cold-working process, a flattening and/or straightening operation can advantageously be carried out. During flattening and/or straightening operations, the permanent set is generally less than 2%, preferably less than about 1%.

An annealing process is optionally performed at a temperature that lies in the range 300° C. to 350° C. The annealing time generally lies in the range 1 to 4 hours. The main purpose of this annealing process is to stabilize the mechanical properties such that they are not altered during a subsequent forming process at a similar temperature. The products according to the invention have the advantage of having very stable mechanical properties at this temperature. Thus, for products whose final thickness of 4 to 6 mm is obtained by hot rolling, the static mechanical property variation is no greater than 10% and preferably no greater than 6% after annealing between 300 and 350° C., and for products whose final thickness of about 2 mm is obtained by cold rolling, the static mechanical property variation is no greater than 40% and preferably no greater than 30% after annealing between 300 and 350° C. Within the scope of the method according to the invention, it is thus possible not to perform a stabilizing annealing process and to immediately carry out forming, in particular for products whose final thickness is obtained by hot rolling. Thanks to the method of the invention, the products according to the invention retain an essentially non-recrystallized grain structure after annealing at between 300 and 350° C.

Sheet metal having a thickness of less than 12 mm obtained by the method of the invention is advantageous, preferably having the following characteristics:

(a) a tensile yield stress measured at 0.2% elongation in the LT direction of at least 250 MPa, and preferably of at least 260 MPa and/or

(b) a tensile yield stress measured at 0.2% elongation in the L direction of at least 260 MPa, and preferably of at least 270 MPa, whereby these properties are achieved even in the case wherein the optional annealing step at a temperature in the range 300° C. to 350° C. is carried out.

Advantageously, sheet metal having a thickness of less than 4 mm obtained by the method of the invention has a tensile yield stress measured at 0.2% elongation in the LT direction of at least 300 MPa, and preferably of at least 320 MPa, whereby these properties are achieved even in the case wherein the optional annealing step at a temperature in the range 300° C. to 350° C. is carried out.

The sheet metal according to the invention preferably has advantageous toughness properties, in particular:

(c) a toughness K_(R)60, measured on specimens of type CCT760 in the L-T direction (where 2ao=253 mm), for an effective crack growth Δa_(eff) of 60 mm, of at least 155 MPa^(√{square root over (m)}), and preferably of at least 165 MPa^(√{square root over (m)}) and/or

(d) a toughness K_(R)60, measured on specimens of type CCT760 in the T-L direction (where 2ao=253 mm), for an effective crack growth Δa_(eff) of 60 mm, of at least 160 MPa^(√{square root over (m)}), and preferably of at least 170 MPa^(√{square root over (m)}).

Preferably, for the products according to the invention, the toughness KR in the T-L direction is greater than that in the L-T direction.

Preferably, the toughness Kapp, measured on specimens of type CCT760 in the T-L direction (where 2ao=253 mm), is at least 125 MPa, and preferably at least 130 MPa. The products according to the invention can be formed at a temperature that lies in the range 300° C. to 350° C. in order to obtain structural elements for an aeroplane, preferably fuselage elements.

Aircraft fuselage elements according to the invention are advantageous because they have

-   -   (a) a tensile yield stress measured at 0.2% elongation in the LT         direction of at least 250 MPa, and preferably of at least 260         MPa and/or     -   (b) a tensile yield stress measured at 0.2% elongation in the L         direction of at least 260 MPa, and preferably of at least 270         MPa.

EXAMPLES Example 1

A plurality of slabs with a thickness of 400 mm were cast, the composition whereof is provided in Table 1.

TABLE 1 Composition in wt % (analyzed by spark optical emission spectrometry, S-OES). Si Fe Cr Mn Mg Zn Ti Zr Sc A 0.02 0.05 <0.01 0.62 4.05 0.28 0.023 0.10 0.19 B 0.02 0.04 <0.01 0.59 3.99 0.29 0.038 0.10 0.19

The slab made of alloy A was homogenized for 5 h at 445° C., whereas the slab made of alloy B was homogenized for 15 h at 515° C. The slabs thus homogenized were hot rolled immediately after homogenization with a hot-rolling starting temperature of 415° C. for slab A and 480° C. for slab B, in order to obtain 4 mm thick sheets.

The tensile static mechanical properties of the sheet made of alloy A remained high, both in the hot-rolled temper (HR) and in the annealed temper (annealing treatment for 4 h at 325° C.), whereas those of the sheet made of alloy B deteriorated after annealing.

TABLE 2 Static mechanical properties obtained for the different sheet in the hot-rolled temper (HR) and in the annealed temper (4 h at 325° C.). Alloy A sheet Alloy B sheet Thickness 4 mm Thickness 4 mm HR Annealing HR Annealing Rp0.2 L, MPa 303 289 287 233 Rm L, MPa 400 393 364 352 A L, % 14.5 16.2 14.8 17.6 Rp0.2 LT, MPa 311 292 276 238 Rm LT, MPa 396 387 361 349 A LT, % 17.7 19.5 18.2 23.0 Kapp MPa√m L-T 129.9 129.1 128.5 Kapp MPa√m T-L 134.9 134.0 125.8 Kr60 MPa√m L-T 172.9 171.5 171.2 Kr60 MPa√m T-L 178.9 177.1 164

The 4-mm sheets were cold rolled to a thickness of 2 mm by three passages, without intermediate heat treatment, then underwent flattening. Different heat treatments were carried out after cold rolling. The tensile mechanical test results are shown in table 3.

TABLE 3 Static mechanical properties obtained for the different cold-rolled sheets having undergone annealing under different conditions. Alloy A sheet Alloy B sheet Annealing Thickness 2 mm Thickness 2 mm after cold Rp02 Rp02 rolling (LT) Rm (LT) A % LT (LT) Rm (LT) A % LT — 417 466 9.95 358 422 10.5 2 h 275° C. 349.5 415 19 256 355 18.2 2 h 325° C. 333 405 21.7 168 311 23.0 2 h 375° C. 297.5 393 21.4 156 301 23.1

The grain structure of the sheets was observed after metallographic etching of the anodic oxidation type under polarized light after cold rolling (CR) or after cold rolling and annealing for 2 h at 325° C.

A qualitative assessment of the microstructure was carried out:

Table 4 shows the results of the microstructural observations of the sheets of compositions A and B in the unwrought cold rolling temper and after annealing treatment (2 h at 325° C.).

TABLE 4 Microstructure (plane LxTC, at mid-thickness) of the sheets Alloy Reference Microstructure A CR Appreciably non-recrystallized 2 h 325° C. Appreciably non-recrystallized B CR Appreciably non-recrystallized 2 h 325° C. Recrystallized

Alloy A according to the invention has excellent recrystallisation resistance.

Example 2

This example studied the effect that the homogenization conditions before hot working have on the mechanical properties. Blocks made of alloy A of dimensions 250×180×120 mm were hot rolled under different conditions until obtaining a thickness of 8 or 12 mm. The conditions are described in Table 5.

TABLE 5 Transformation conditions of the different blocks made of alloy A Homogenization Initial rolling Final Final rolling temperature Homogenization T(eq) temperature thickness temperature (° C.) duration (h) at 400° C. (° C.) (mm) (° C.) CD2 450 15 298 440 12 329 CD3 400 15 15 390 12 319 CD4 450 15 298 440 8 325 CF1 450 5 99 440 8 330 CF2 450 5 99 12 327 CF3 400 5 5 405 12 320 CF4 515 17 9341 8 325

The mechanical properties were measured on the sheets having undergone rolling or a treatment. The results are presented in Table 6.

TABLE 6 Static mechanical properties obtained for the different sheets in the hot rolled temper (HR) and in the annealed temper (4 h at 325° C.). Annealing for 4 h HR at 325° C. Rp0.2 Rm A Rp0.2 Rm A block direction MPa MPa % MPa MPa % CD2 L 251 377 15.4 243 370 16.0 CD3 L 286 398 14.5 278 391 15.4 CD4 L 260 371 13.6 252 366 16.7 CF1 L 275 381 16.1 267 373 17.1 CF2 L 268 390 12.9 262 382 13.8 CF3 L 288 399 14.8 280 392 15.4 CF4 L 223 341 15.7 209 339 17.3

The products obtained by the method according to the invention (CD3, CF1, CF2, CF3) have advantageous mechanical properties, in particular Rp0.2 in the L direction of at least 260 MPa after hot rolling and after annealing for 4 h at 325° C. 

1. Method for producing a wrought product made of an aluminum alloy comprising: a) Producing a molten metal bath having an aluminum base, comprising, in wt %, Mg: 3.8-4.2; Mn: 0.3-0.8 and optionally 0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.01-0.05 and optionally 0.015-0.030; Zr: 0.07-0.15 and optionally 0.08-0.12; Cr: <0.01; Fe: <0.15; Si<0.1; other elements <0.05 each and <0.15 combined, the remainder being aluminum; b) Casting an unwrought product from said metal bath; c) Homogenizing said unwrought product at a temperature that lies in the range 370° C. to 450° C., for a duration that lies in the range 2 to 50 hours such that the equivalent time at 400° C. lies in a range 5 to 100 hours, the equivalent time t(eq) at 400° C. being defined by formula: ${t({eq})} = \frac{\int{{\exp \left( {{- 29122}/T} \right)}{dt}}}{\exp \left( {{- 29122}/T_{ref}} \right)}$ where T is the current temperature expressed in Kelvin, which changes over time t (in hours) and Tref is a reference temperature of 400° C. (673 K), t(eq) being expressed in hours, the constant Q/R=29122 K being derived from the activation energy for the diffusion of Zr, Q=242000 J/mol, d) Hot-working the unwrought product thus homogenized is hot worked with an initial temperature in the range 350° C. to 450° C. and is optionally cold-worked; e) a flattening and/or straightening process is optionally carried out; f) an annealing process is optionally carried out at a temperature that lies in the range 300° C. to 350° C.
 2. Method according to claim 1, wherein the homogenization duration lies in a range 5 to 30 hours.
 3. Method according to claim 1, wherein working is carried out by rolling in order to obtain a sheet metal and wherein a final thickness of a sheet obtained is less than 12 mm.
 4. Method according to claim 1, wherein working is carried out by extrusion in order to obtain a profile.
 5. Wrought product made of an aluminum alloy having the composition, in wt %, Mg: 3.8-4.2; Mn: 0.3-0.8 and optionally 0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.01-0.05 and optionally 0.015-0.030; Zr: 0.07-0.15 and optionally 0.08-0.12; Cr: <0.01; Fe: <0.15; Si<0.1; other elements <0.05 each and <0.15 combined, the remainder being aluminum, obtainable by the method according to claim
 1. 6. Wrought product in the form of a sheet having a thickness of less than 12 mm, obtainable by the method according to claim 3, wherein (a) the tensile yield stress thereof measured at 0.2% elongation in the LT direction of at least 250 MPa, and optionally of at least 260 MPa and/or (b) the tensile yield stress thereof measured at 0.2% elongation in the L direction of at least 260 MPa, and optionally of at least 270 MPa.
 7. Wrought Product in the form of a Sheet according to claim 6, wherein (c) the toughness K_(R)60 thereof, measured on specimens of type CCT760 in the L-T direction (where 2ao=253 mm), for an effective crack growth Δa_(eff) of 60 mm, of at least 155 MPa^(√{square root over (m)}), and optionally of at least 165 MPa^(√{square root over (m)}) and/or (d) the toughness K_(R)60 thereof, measured on specimens of type CCT760 in the T-L direction (where 2ao=253 mm), for an effective crack growth Δa_(eff) of 60 mm, of at least 160 MPa^(√{square root over (m)}), and optionally of at least 170 MPa^(√{square root over (m)}).
 8. Method according to claim 1, wherein, at the end of f, forming is carried out at a temperature that lies in the range 300° C. to 350° C.
 9. Aircraft fuselage element obtainable according to the method according to claim 8, wherein (a) the tensile yield stress thereof measured at 0.2% elongation in the LT direction of at least 250 MPa, and optionally of at least 260 MPa and/or (b) the tensile yield stress thereof measured at 0.2% elongation in the L direction of at least 260 MPa, and optionally of at least 270 MPa. 